Tine fixation components for implantable medical devices

ABSTRACT

A tine portion for an implantable medical device includes a hook segment and a distal segment terminated by a tissue-piercing tip, wherein the distal segment extends from the hook segment to the tip. The hook segment, which is elastically deformable from a pre-set curvature, has one of: a round cross-section and an elliptical cross-section, while the distal segment has a flattened, or approximately rectangular cross-section. One or a pair of the tine portions may be integrally formed, with a base portion, from a superelastic wire, wherein the base portion is configured to fixedly attach to the device, for example, being captured between insulative members of a fixation subassembly.

RELATED APPLICATION

The application is a continuation of U.S. patent application Ser. No.14/831,417, filed Aug. 20, 2015 entitled “TINE FIXATION COMPONENTS FORIMPLANTABLE MEDICAL DEVICES”, which is a divisional of U.S. patentapplication Ser. No. 13/955,127, filed Jul. 3, 2013 entitled “TINEFIXATION COMPONENTS FOR IMPLANTABLE MEDICAL DEVICES”, both of which areherein incorporated by reference in their entirety.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is related to the commonly-assigned U.S.application Ser. No. 13/955,393 and, U.S. Pat. No. 9,155,882, which arefiled concurrently herewith and incorporated by reference, in theirentirety.

TECHNICAL FIELD

The present invention pertains to implantable medical devices, and, morespecifically, to tissue-penetrating fixation components andsubassemblies thereof.

BACKGROUND

An implantable medical device, for the delivery of stimulation therapyand/or for diagnostic sensing, may include at least onetissue-penetrating fixation component configured to hold the device atan implant location. FIG. 1 is a schematic diagram that shows potentialcardiac implant sites for such a device, for example, within anappendage 102 of a right atrium RA, within a coronary vein CV (via acoronary sinus ostium CSOS), or in proximity to an apex 103 of a rightventricle RV. FIG. 2 is a plan view of an exemplary implantable medicaldevice 200, which includes a tissue-penetrating fixation componentformed by a plurality of tine portions 230. FIG. 2 further illustratesdevice 200 including a hermitically sealed housing 220 that containscontrol electronics and a power source (not shown), and which defines alongitudinal axis 2 of device 200. Housing 220 may be formed from amedical grade stainless steel or titanium alloy and have an insulativelayer formed thereover, for example, parylene, polyimide, or urethane.With further reference to FIG. 2, device 200 includes a pair ofelectrodes 261, 262, which may form a bipolar pair for cardiac pacingand sensing; tine portions 230 surround electrode 261 and are configuredto penetrate tissue in order to hold electrode 261 in intimate contactwith tissue, for example, at one of the aforementioned implant sites,while securing, or fixating device 200 for chronic implantation at thesite. Further description of a suitable construction for device 200 maybe found in the co-pending and commonly assigned United States PatentApplication having the pre-grant publication number 2012/0172690 A1.

With reference to FIG. 3A, device 200 may be delivered to an implantlocation via a delivery catheter 300. For example, with reference toFIG. 1, if the target implant site is located in the right atrium RA,coronary vein CV, or right ventricle RV, a distal end 310 of catheter300 may be maneuvered into the heart through a superior vena cava SVC oran inferior vena cava IVC, according to a transvenous delivery methodknown in the art. FIG. 3A shows a partial cross-section of distal end310 of catheter 300, which is formed like a cup to hold and containdevice 200 for delivery to the implant site. FIG. 3A illustrates device200 having been loaded into distal end 310 so that a hook segment 231 ofeach tine portion 230 is elastically deformed, from a pre-set curvaturethereof, to an open position, at which a distal segment 232 of each tineportion 230 extends distally toward an opening 313 of catheter distalend 310. Each tine portion 230 is preferably formed from a superelasticmaterial, such as Nitinol (a metal alloy of nickel and titanium). FIG.3A further illustrates a deployment element 320 abutting a proximal endof device 200 and extending proximally therefrom, through a lumen ofcatheter 300, and out from a proximal opening 301 thereof. Element 320may be moved, per arrow M, by an operator to push device 200, per arrowP, out from opening 313 of distal end 310, for example, when opening 313has been located by the operator in close proximity to tissue at thetarget implant site.

FIG. 3B, is an enlarged view of distal segment 232 of one of tineportions 230, wherein a tissue-piercing tip 322, which terminates distalsegment 232, has just been pushed out through opening 313 of distal end310 of catheter 300 and into contact with tissue T. FIG. 3B illustratesdistal segment 232 supported by the surrounding wall of distal end 310,in proximity to opening 313, so that the push force of deploymentelement 320 is effectively transferred through tip 322 to first compressthe tissue T, as shown, and then to pierce the tissue T for penetrationtherein, which is shown in FIGS. 3C-D. FIGS. 3C-D illustrate partialtine penetration and full tine penetration, respectively, as deploymentelement 320 continues to push device 200 out opening 313. It can be seenthat the elastic nature of each tine portion 230, once the constraint ofthe distal end 310 is withdrawn, allows the corresponding hook segment231 to relax back toward the pre-set curvature thereof within thetissue. The full penetration of tine portions 230, shown FIG. 3D, isrepresentative of acute fixation of device 200 at the implant site, forexample, for the evaluation of device performance (e.g., pacing andsensing via electrodes 261, 262). It should be noted that, at someimplant sites, tine portions 230 may, at full penetration, extend backout from tissue T, for example, generally toward distal end 310 ofcatheter 300.

With further reference to FIG. 3D, a tether 350 is shown looping throughan eye feature 205 formed at the proximal end of device 200; tether 350extends proximally through a lumen of deployment element 320 to aproximal end 351 thereof, outside a proximal end of deployment element320, which may be seen in FIG. 3A. Thus, if the performance of acutelyfixated device 200 is unsatisfactory, the operator may use tether 350 topull device 200 back into distal end 310, thereby withdrawing tineportions 230 from the tissue, so that device may be moved by deliverycatheter 300 to another potential implant site. Alternately, if theacutely fixated device 200 performs satisfactorily, proximal end 351 oftether 350 may be severed to pull tether 350 out from eye feature 205 ofdevice 200, and the fully penetrated tine portions 230 continue tofixate device 200 for chronic implant.

The aforementioned co-pending and commonly assigned Unite States PatentApplication '690 discloses suitable embodiments of a fixation componenthaving tine portions similar to tine portions 230, wherein the tineportions exhibit a suitable baseline performance, for example, in termsof a deployment force, an acute retraction force (for repositioning),atraumatic retraction, and acute and chronic fixation forces. Yet, thereis still a need for new configurations of tine portions for implantabledevices, like device 200, that may further enhance fixation.

SUMMARY

Embodiments of the present invention encompass implantable medicaldevices (e.g., cardiac pacemakers) and tissue-penetrating fixationcomponents and subassemblies thereof, which include one or more tineportions configured for increased strain relief during the flexingthereof, either at initial implant (particularly in cases where theretraction of penetrated tines is necessary for repositioning thedevice), or when subject to cyclic loading during a chronic implant ofthe fixated device, for example, within a beating heart.

According to some preferred embodiments, a tine portion of atissue-penetrating component of an implantable medical device includes ahook segment and a distal segment, wherein the hook segment has a roundcross-section, and the distal segment has a flattened cross-section. Thehook segment is pre-set to extend along a curvature that encloses anangle of between 135 degrees and 270 degrees, from a proximal endthereof to a distal end thereof, and which is elastically deformablefrom the pre-set curvature to an open position; and the distal segmentis pre-set to extend along a relatively straight line, approximatelytangent to the distal end of the hook segment. In some embodiments oftissue-penetrating components, one or a pair of tine portions areintegrally formed, with a base portion, from a superelastic wire (e.g.,Nitinol). A fixation subassembly for an implantable medical device,according to some embodiments, includes insulative members, betweenwhich the base portion(s) of one or a pair of the tissue-penetratingcomponents is/are captured.

BRIEF DESCRIPTION OF THE DRAWINGS

The following drawings are illustrative of particular embodiments of thepresent invention and therefore do not limit the scope of the invention.The drawings are not to scale (unless so stated) and are intended foruse in conjunction with the explanations in the following detaileddescription. Embodiments will hereinafter be described in conjunctionwith the appended drawings wherein like numerals/letters denote likeelements, and:

FIG. 1 is a schematic diagram showing potential implant sites forembodiments of the present invention;

FIG. 2 is a plan view of an exemplary implantable medical device;

FIG. 3A is a plan view of the medical device loaded in a deliverycatheter, according to some embodiments, wherein tine portions of atissue-penetrating fixation component thereof are elastically deformedinto an open position;

FIG. 3B is an enlarged detail view of one of the tine portions initiallycontacting tissue at an implant site;

FIGS. 3C-D are plan views of the device and catheter in subsequent stepsof implanting the device, when the tine portions have penetrated thetissue;

FIG. 4A is a schematic representation of a flexing tine portion;

FIG. 4B is a perspective view of a tissue-penetrating fixationcomponent, according to some embodiments of the present invention;

FIG. 5A is a plan view of an implantable medical device, according tosome embodiments;

FIG. 5B is an exploded perspective view of the implantable medicaldevice, according to some embodiments;

FIGS. 6A-C are elevation views of tissue-penetrating fixationcomponents, according to some alternate embodiments;

FIG. 7A is an exploded perspective view of a fixation subassembly,according to some embodiments; and

FIG. 7B is a top plan view of portions of the fixation subassemblypartially assembled.

DETAILED DESCRIPTION

The following detailed description is exemplary in nature and is notintended to limit the scope, applicability, or configuration of theinvention in any way. Rather, the following description providespractical examples, and those skilled in the art will recognize thatsome of the examples may have suitable alternatives.

FIG. 4A is a schematic representation of one of tine portions 230isolated from the above-described implantable medical device 200,wherein exemplary flexing, per arrows F1, F2, of tine portion 230 isillustrated. Such flexing may be encountered by tine portion 230, oncetine portion 230 has penetrated tissue to fix device 200 at a chronicimplant site for cardiac monitoring and/or therapy, for example, asillustrated in FIG. 3D. Thus, fatigue life is a considerationinfluencing the configuration of tine portions for those implantablemedical devices that may be subjected to cyclic loading caused byhundreds of millions of heart beats, over the life of their implant. InFIG. 4A, a zone of stress concentration SC, for example, in response tothe flexing per arrows F1, F2, is circled; zone SC is located inproximity to a proximal end 31 of hook segment 231 of tine portion 230,where hook segment 231 joins with a base portion 203. Base portion 203and tine portion 230 may be integrally formed, wherein base portion 203is configured to be fixedly attached to device 200. Stress concentrationin zone SC may also result from deformation of hook segment 231 into theopen position (FIG. 3A), for example, upon initial loading of device 200and retraction of device 200 back into distal end 310 of catheter forrepositioning, which, in combination with the repeated force ofdeployment, can potentially push tine portion 230 toward an elasticlimit and may make tine portion 230 subsequently more vulnerable tofatigue under the aforementioned cyclic loading. Although rounded edgesof tine portions 230 effectively reduce the concentration of stress, aspreviously described in the aforementioned commonly-assigned UnitedStates Patent Application '690, embodiments of the present inventionincorporate tine portions in which the hook segments thereof have roundcross-sections, for example, as illustrated in FIG. 4B, as analternative means of alleviating the stress concentration.

FIG. 4B is a perspective view of a tissue-penetrating fixation component43, which includes a tine portion 430, according to some embodiments,one or more of which may be integrated into an implantable medicaldevice 500, which is described below in conjunction with FIGS. 5A-B. Abase portion 403 is shown integrally formed with tine portion 430,according to some preferred embodiments, wherein base portion 403 isconfigured for attachment to device 500. FIG. 4B illustrates a hooksegment 431 of tine portion 430 extending from a proximal end 41thereof, in proximity to base portion 403, to a distal end 42 thereof,in proximity to a distal segment 432 of tine portion 430, wherein hooksegment 431 has a round cross-section (dashed line). FIG. 4B furtherillustrates distal segment 432 having a flattened cross-section, or anapproximately rectangular profile, for example, similar to that ofdistal segment 232, wherein the edges spanning a thickness t thereof arerounded. According to an exemplary embodiment, the round cross-sectionof hook segment 432 has a diameter of approximately 0.007 inch toapproximately 0.012 inch, thickness t of distal segment 432 isapproximately 0.005 inch, and a width W of distal segment 432 isapproximately 0.025 inch. It should be noted that, according to somealternate embodiments, hook segment 432 may have an ellipticalcross-section, for example, having a major axis of approximately 0.012inch and a minor axis of approximately 0.007 inch. Tine portion 430 andbase portion 403 of component 43, according to some preferredembodiments, are integrally formed from a superelastic wire, forexample, a medical grade Nitinol wire, wherein a wire forming processand a rolling or swaging process may be employed to manufacturecomponent 43.

FIG. 5A is a plan view of implantable medical device 500; and FIG. 5B isan exploded perspective view of device 500, according to someembodiments. FIGS. 5A-B illustrates device 500 including a hermiticallysealed housing 520 and a pair of electrodes 561, 562; housing 520defines a longitudinal axis 5 of device 500, and contains controlelectronics and a power source (not shown), which, for example, togetherwith electrodes 561, 562, are adapted for cardiac pacing and sensing.FIG. 5B further illustrates tine portions 430 included in fixationsubassembly 745, which is configured for attachment to housing 520, viatabs 73 formed in a distal end 501 thereof, and illustrates electrode561 included in an electrode subassembly 560, which is configured to fittogether with fixation subassembly 745, according to some embodiments.Tine portions 430, like tine portions 230 described above, penetratetissue in order to secure device 500 at an implant site, for example, acardiac site in the right atrium RA or the right ventricle RV (FIG. 1),having been deployed from distal end 310 of delivery catheter 300 (FIGS.3A-D).

With further reference to FIG. 5B, electrode subassembly 560 alsoincludes steroid contained in a monolithic controlled release device(MCRD) 564, such as is known to those skilled in the art, which isconfigured to fit within a central recess of electrode 561; electrode561 and MCRD 564 are sandwiched together between a cap 566 and aninterlock interface component 563 of subassembly 560, when withelectrode subassembly 560 and fixation subassembly 745 are assembledtogether at distal end 501 of housing 520. An hermetic feedthroughsubassembly (not shown) for electrode 561, the construction of which isknown to those skilled in the art, may extend into housing at thelocation designated with reference numeral 506. The aforementionedcommonly-assigned United States Patent Application '690 describes ingreater detail an exemplary means by which electrode 561 may beintegrated into device 500, as well as additional construction detailsfor an implantable medical device like device 500.

According to some preferred embodiments, adjacent pairs of tine portions430 may be integrally formed together, for example, in a tissuepenetrating component 63A, which is shown in FIG. 6A. FIGS. 6A-C areelevation views of various tissue-penetrating fixation components 63A-C,according to some alternate embodiments of the present invention. FIG.6A illustrates component 63A including a base portion 603 that extendsbetween proximal ends 41 of hook segments 431 of tine portions 430.According to the illustrated embodiment, base portion 603 extends alongan arc (best seen in FIGS. 7A-B) that is centered on a longitudinal axis6A of component 63A, wherein axis 6A approximately coincides withlongitudinal axis 5 of device 500, when component 63A is included insubassembly 745. Likewise, FIGS. 6B-C illustrate each tissue-penetratingcomponent 63B-C including a pair of tine portions 630A-B andcorresponding base portion 603, which extends along an arc that iscentered on a corresponding longitudinal axis 6B-C of the correspondingcomponent 63B-C. As described above, tine portions 430, 630B-C and baseportion 603 of each component 63A-C are preferably integrally formedfrom a superelastic wire, for example, Nitinol wire, wherein baseportion 603 and hook segments 431, 631B-C have a round profile, whiledistal segments 432, 632B-C have a flattened profile. Alternately, thewire, from which tine portions 430, 630B-C are formed, may have anelliptical cross-section so that base portion 603 and hook segments 431,631B-C likewise have an elliptical cross-section. Any suitable wireforming process known to those skilled in the art, for example, in themanufacture of spring component, may be employed to form components63A-C, wherein a rolling swaging process may be employed to form theflattened cross-section of distal segments 432, 632B-C. Furthermore,according to some methods, a secondary stamping or cutting process maybe employed to further form distal segments 432, 632B-C.

With reference to FIGS. 6A-C, components 63A-C differ from one anotheraccording to the pre-set curvatures of corresponding hook segments 431,631B, 631C, as follows. According to FIG. 6A, each hook segment 431 ispre-set to extend along a curvature that encloses an angle Θ of between180 degrees and 270 degrees, from proximal end 41 to distal end 42, suchthat the corresponding distal segment 432 extends from distal end 42,along a relatively straight line (approximately tangent to distal end42) toward longitudinal axis 6A. According to FIG. 6B, each hook segment631B is pre-set to extend along a curvature that encloses an angle β ofbetween 135 degrees and 180 degrees, preferably approximately 160degrees, from proximal end 61B to distal end 62B, such that thecorresponding distal segment 632B, extends from distal end 62B, along arelatively straight line (approximately tangent to distal end 62B) awayfrom longitudinal axis 6B. According to FIG. 6C, each hook segment 631Cis pre-set to extend along a curvature that encloses an angle φ of 180degrees, from proximal end 61C to distal end 62C, such that thecorresponding distal segment 632C, extends from distal end 62C, along arelatively straight line (approximately tangent to distal end 62C) thatis approximately parallel to longitudinal axis 6B.

With further reference to FIGS. 6A-C, it may be noted that distalsegments 632B, 632C of respective tine portions 630B, 630C arerelatively short compared to distal segment 432 of each tine portion430, for example, being between approximately 0.05 inch andapproximately 0.1 inch. The shorter length of distal segments 632B-C mayhelp to prevent perforation through the wall of a structure, forexample, the heart, at some implant locations, when tine portions630B-C, having been deformed into the open position for loading into adelivery catheter, for example, catheter 300 (FIG. 3A), are initiallypushed into a target implant site (FIG. 3B-D). The shorter length mayalso help reduce a probability for penetrated tine portions 630B-C tointerfere with blood vessels, for example, compressing or pinching thevessels. Furthermore, the pre-set curvature (defined by theaforementioned angles β, φ) of hook segments 631B-C, in combination withthe shorter length of distal segments 632B-C, can help to maintain aneffective orientation of distal segments 632B-C for tissue penetration,when the corresponding hook segments 631B-C are deformed into the openposition within distal end 310 of delivery catheter 300 in a mannersimilar to that illustrated in FIG. 3A.

With further reference to FIG. 6A, dashed lines illustrate an alternateconfiguration of hook segment 431, in proximity to proximal end 41thereof. According to the alternate configuration, an extension ofsegment 431 from proximal end 41 toward axis 6A increases an overall arclength of tine portion 430 for added flexibility in elasticallydeforming hook segment 431 to the open position, for example, duringretraction into catheter distal end 310 (FIGS. 3A-C). Although notshown, each hook segment 631B, 61C of the corresponding tine portion630B, 630C (FIGS. 6B-C) may also be configured to extend from thecorresponding proximal end 61B, 61C back toward the corresponding axis6B, 6C, according to some alternate embodiments.

FIG. 7A is an exploded perspective view of fixation subassembly 745,according to some embodiments; and FIG. 7B is a top plan view ofportions of fixation subassembly 745 partially assembled. FIG. 7Aillustrates fixation subassembly 745 including an insulative lockingmember 750, an insulative retainer 740, and a pair of thetissue-penetrating fixation components 63A, which may be exchanged for apair of either of the components 63B, 63C, according to some alternateembodiments. FIG. 7A further illustrates locking member 750 including acollar 756 and a body 755 extending therefrom, wherein collar 756includes a plurality of notches 751, which are formed in an outerperimeter surface thereof, and are spaced apart from one another aboutthe perimeter. Retainer 740 is shown including a groove 746 formedtherein, to receive base portions 603 of components 63A, and an internalsurface 76, which defines an opening 765 to receive body 755 of lockingmember 750. According to an exemplary embodiment, retainer 740 andlocking member 750 are formed, for example, by injection molding, from arelatively hard biocompatible plastic, such as polyether ether ketone(PEEK). FIG. 7B illustrates base portions 603 of fixation components 63Aseated in groove 746 of retainer 740, such that tine portions 430 arespaced apart from one another, about the perimeter of retainer 740, byapproximately 90 degrees. With further reference to FIG. 7A, notches 751are similarly spaced about a perimeter of locking member 750, so that,when components 63A are seated in groove 746 of retainer 740, and body755 of locking member 750 is received in opening 765 of retainer 740,each hook segment 431 extends through a corresponding notch 751. Itshould be noted that, according to some alternate embodiments, whereinthree pair of tine portions 430 are employed, the spacing thereof, alongwith corresponding notches 751, about the perimeter of retainer 740, isapproximately 60 degrees.

With reference back to FIG. 5B, in conjunction with FIG. 7A, accordingto some embodiments, locking member 750 further includes an internalsurface with interlocking features 753 formed therein. According to theillustrated embodiment, features 753 are configured to mate with tabs73, for example, by inserting each tab 73 through a slot adjacent eachfeature 753 and then rotating subassembly 745 to engage a ramped surfaceof each feature 753 beneath a corresponding tab 73. With furtherreference to FIG. 5B, interlock interface component 563 of electrodesubassembly 560, may, when assembled together with fixation subassembly745, engage within the slots to retain features 753 in engagement withtabs 73.

In the foregoing detailed description, the invention has been describedwith reference to specific embodiments. However, it may be appreciatedthat various modifications and changes can be made without departingfrom the scope of the invention as set forth in the appended claims.

The invention claimed is:
 1. A tissue-penetrating fixation component foran implantable medical device comprising: a base portion beingconfigured to be fixedly attached to the medical device; and a pluralityof tine portions integrally formed with the base portion, each of theplurality of tine portions comprising: a hook segment having one of around cross section that has a diameter and a continuous curvaturearound the round cross section or an elliptical cross-section that has amajor axis and a minor axis and a continuous curvature around theelliptical cross-section, the hook segment being pre-set to extend alonga curvature from a proximal end thereof, in proximity to the baseportion, to a distal end thereof, the hook segment also beingelastically deformable from the pre-set curvature to an open position;and a distal segment that is separate from the hook segment, the distalsegment being terminated by a tissue-piercing tip, the distal segmenthaving a flattened cross-section and being pre-set to extend along aline that is tangent to the distal end of the hook segment, from thedistal end of the hook segment to the tip.
 2. The component of claim 1,wherein the angle enclosed by the pre-set curvature of the hook segmentis greater than 180 degrees.
 3. The component of claim 1, wherein theangle enclosed by the pre-set curvature of the hook segment is 180degrees.
 4. The component of claim 1, wherein the angle enclosed by thepre-set curvature of the hook segment is less than 180 degrees.
 5. Thecomponent of claim 1, wherein the base portion and the plurality of tineportions are integrally formed from a superelastic material.
 6. Thecomponent of claim 5, wherein the superelastic material comprisesNitinol.
 7. The component of claim 1, wherein the base portion extendsbetween the proximal ends of the hook segments.
 8. The component ofclaim 7, wherein the base portion extends along an arc.
 9. The componentof claim 1, wherein the plurality of tine portions comprises four tineportions spaced apart from one another by 90 degrees.
 10. An implantablemedical device comprising: a hermetically sealed housing; and atissue-penetrating fixation member comprising: a base portion beingconfigured to be fixedly attached to the medical device; and a pluralityof tine portions integrally formed with the base portion, each of theplurality of tine portions comprising: a hook segment having one of around cross section that has a diameter and a continuous curvaturearound the round cross section or an elliptical cross-section that has amajor axis and a minor axis and a continuous curvature around theelliptical cross-section, the hook segment being pre-set to extend alonga curvature from a proximal end thereof, in proximity to the baseportion, to a distal end thereof, the hook segment also beingelastically deformable from the pre-set curvature to an open position;and a distal segment that is separate from the hook segment, the distalsegment being terminated by a tissue-piercing tip, the distal segmenthaving a flattened cross-section and being pre-set to extend along aline that is tangent to the distal end of the hook segment, from thedistal end of the hook segment to the tip.
 11. The device of claim 10,wherein the angle enclosed by the pre-set curvature of the hook segmentis greater than 180 degrees.
 12. The device of claim 10, wherein theangle enclosed by the pre-set curvature of the hook segment is 180degrees.
 13. The device of claim 10, wherein the angle enclosed by thepre-set curvature of the hook segment is less than 180 degrees.
 14. Thedevice of claim 10, wherein the base portion and the plurality of tineportions are integrally formed from a superelastic material.
 15. Thedevice of claim 14, wherein the superelastic material comprises Nitinol.16. The device of claim 10, wherein the base portion extends between theproximal ends of the hook segments.
 17. The device of claim 16, whereinthe base portion extends along an arc.
 18. The device of claim 10,wherein the plurality of tine portions comprises four tine portionsspaced apart from one another by 90 degrees.